1. Field of the Invention
This invention relates generally to switching circuits, and, more particularly, to means for reducing ripple currents that arise in such circuits.
2. Description of the Related Art
Many electronic circuits include signals which are created by a periodic switching means. For example, a switching voltage regulator creates an output voltage by controlling the flow of current through an inductor with a switching circuit. This switching can give rise to ripple voltages in one or more of the circuit's signals, which are generally undesirable.
One circuit which is particularly prone to ripple voltages is the charge pump. Charge pumps are electronic circuits that use capacitors as energy storage elements to convert DC voltages into other DC voltages; switches are used to control the connection of voltages to one or more capacitors. For instance, to generate a higher voltage, during a ‘charging’ phase, a capacitor is connected across a voltage and charged up. In a ‘transfer’ phase, the capacitor is disconnected from the original charging voltage and reconnected with its negative terminal to the original positive charging voltage. Because the capacitor retains the voltage across it (ignoring leakage effects), the positive terminal voltage is added to the original, effectively doubling the voltage. Note that a charge pump could alternatively be configured to provide a negative output voltage.
A basic charge pump is shown in FIG. 1a. During the charging phase, switches S2 and S3 are closed and S1 and S4 are open, and capacitor C is charged to a voltage VC which is approximately equal to VDD. During the transfer phase, switches S2 and S3 are opened and S1 and S4 are closed, making the voltage at the output terminal OUT equal to VDD+VC≈2*VDD.
A ‘reservoir’ capacitor CR is typically connected across the output to smooth out the output waveform. However, due to the switching required to operate the charge pump, there is still a considerable ripple present in the output voltage. When the charge pump output is used to, for example, provide a supply voltage for an operational amplifier, this output ripple can feed through to and unacceptably degrade the output of the amplifier.